Fault-current limiters have been implemented using superconducting material to carry the current in an electricity supply network. Such networks normally are expected to carry currents of a few hundred amperes, but if a short-circuit (fault) occurs, the current rises to levels which can be several tens of thousands of amperes. When a fault occurs, the current density in the superconducting material exceeds the critical current density of the material, which ceases to be superconducting and becomes resistive. This process is known as quenching. The presence of resistance in the circuit causes the current to be reduced, or “limited”, reducing the potentially damaging effects of excessively high currents in the network.
Networks for the transmission and distribution of electricity are generally three-phase, so three conductors are required and for each of said three conductors, current-limiting means must be provided. Typically a superconducting fault-current limiter will comprise three current limiters as described above, housed either in a single vessel or in a plurality of vessels.
Some types of superconducting fault current limiters employ a superconducting element together with a semiconductor switching element (and associated circuit elements), which switches the state of the limiter between normal and fault conditions. Others require an iron core magentically linking several superconducting coils. It is known to provide a device for a current limiter, the device comprising at least one coil assembly adapted to carry a current, the coil assembly comprising a first coil, comprising a first superconducting element, adapted to carry a first portion of said current, and a second coil, comprising a second superconducting element, adapted to carry a second portion of said current, wherein said first and second coils are arranged such that, when said first and second superconducting elements are each in a superconducting state and said coil assembly carries said current, a magnetic field generated by said first portion of said current in said first coil is substantially cancelled by a magnetic field generated by said second portion of said current in said second coil.
Examples of this type are disclosed in EP0350916 (U.S. Pat. No. 5,021,914) and WO 2012/093042 (the contents of the latter being incorporated herein by reference).
U.S. Pat. No. 6,337,785 and US2009190274 (granted as U.S. Pat. No. 7,675,719) teach devices in which the first and second superconducting elements have different “quench characteristics” so that one superconducting element has a lower critical current than the other. This is achieved by use of two different superconducting materials for the respective two superconducting elements. US2009190274 employs a semiconductor switch to guarantee the change of state. In the case of U.S. Pat. No. 6,337,785, the two coils are arranged in series and, in the superconducting state, therefore pass the same current, but when a first coil quenches, current routes instead through a parallel shunt resistor, causing a rapid net impedance due to the second coil. This provides a rapid transition of the superconducting elements from the superconducting to the normal state (quenching) when fault current begins to flow, without needing a separate switch element or an iron core. The device may therefore be used in a superconducting fault current limiter to provide a low inductance during normal operation and a rapid quench under fault current conditions.
However, use of different materials complicates the manufacturing process, and limits the materials choices open to the circuit designer. Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art.